Fix service used by dbm between span.service and peer.service

[zarirhamza]

2.0 Upgrade Guide notes

What does this PR do?
Changes configuration to only use peer.service as the propagated dbm service if the service name for the span is nil or the global default value.

This will only trigger when the user enables DD_TRACE_REMOVE_INTEGRATION_SERVICE_NAMES_ENABLED and intends to use peer.service as the distinguishing factor between spans instead of span.service.

Motivation:

Additional Notes:

How to test the change?

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (65f12dd) 98.23% compared to head (3c7169f) 98.23%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3459   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files        1270     1270           
  Lines       74802    74825   +23     
  Branches     3510     3512    +2     
=======================================
+ Hits        73479    73504   +25     
+ Misses       1323     1321    -2     

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